An electrolytic cell decomposes chemical compounds by means of electrical energy, in a process called electrolysis; the Greek word lysis means to break up. The result is that the chemical energy is increased. Important examples of electrolysis are the decomposition of water into hydrogen and oxygen, and bauxite into aluminium and other chemicals
An electrolytic cell has three component parts: an electrolyte and two electrodes (a cathode and an anode). The electrolyte is usually a solution of water or other solvents in which ions are dissolved. Molten salts such as sodium chloride are also electrolytes. When driven by an external voltage applied to the electrodes, the electrolyte provides ions that flow to and from the electrodes, where charge-transferring, or faradaic, or redox, reactions can take place. Only for an external electrical potential (i.e. voltage) of the correct polarity and large enough magnitude can an electrolytic cell decompose a normally stable, or inert chemical compound in the solution. The electrical energy provided undoes the effect of spontaneous chemical reactions.
3. Note that the site of oxidation is still the anode and the site of reduction is still the cathode, but the charge on these two electrodes are reversed. Anode is now charged and the cathode has a – charged.
4. The conditions under which the electrolyte cell operates are very important. The substance that is the strongest reducing agent (the substance with the highest EHYPERLINK “http://www.saskschools.ca/curr_content/chem30/modules/module8/reduction.html”0HYPERLINK “http://www.saskschools.ca/curr_content/chem30/modules/module8/reduction.html” value in the table) will undergo oxidation. The substance that is the strongest oxidizing agent will be reduced. If a solution of sodium chloride (containing water) was used in the above system, hydrogen would undergo reduction instead of sodium, because it is a stronger reducing agent that sodium.
An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt “battery”. (Actually a single “Galvanic cell”; a battery properly consists of multiple cells.
Half-cells The Bunsen cell, invented by Robert Bunsen.
An electrochemical cell consists of two half-cells. Each half-cell consists of an electrode, and an electrolyte. The two half-cells may use the same electrolyte, or they may use different electrolytes. The chemical reactions in the cell may involve the electrolyte, the electrodes or an external substance (as in fuel cells which may use hydrogen gas as a reactant). In a full electrochemical cell,, species from one half-cell lose electrons (oxidation) to their electrode while species from the other half-cell gain electrons (reduction) from their electrode. A salt bridge (i.e. filter paper soaked in KNO3) is often employed to provide ionic contact between two half-cells with different electrolytes-to prevent the solutions from mixing and causing unwanted side reactions. As electrons flow from one half-cell to the other, a difference in charge is established. If no salt bridge were used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate. Other devices for achieving separation of solutions are porous pots and gelled solutions. A porous pot is used in the Bunsen cell (right).
Equilibrium reaction Each half-cell has a characteristic voltage. Different choices of substances for each half-cell give different potential differences. Each reaction is undergoing an equilibrium reaction between different oxidation states of the ions-when equilibrium is reached the cell cannot provide further voltage. In the half-cell which is undergoing oxidation, the closer the equilibrium lies to the ion/atom with the more positive oxidation state the more potential this reaction will provide. Similarly, in the reduction reaction, the further the equilibrium lies to the ion/atom with the more negative oxidation state the higher the potential.
Electrode potential The cell potential can be predicted through the use of electrode potentials (the voltages of each half-cell). The difference in voltage between electrode potentials gives a prediction for the potential measured.
Cell potentials have a possible range of about zero to 6 volts. Cells using water-based electrolytes are usually limited to cell potentials less than about 2.5 volts, because the very powerful oxidizing and reducing agents which would be required to produce a higher cell potential tend to react with the water.
Electrochemical cell types Main types
Cells are classified into two broad categories,
Primary cells irreversibly (within limits of practicality) transform chemical energy to electrical energy. When the initial supply of reactants is exhausted, energy cannot be readily restored to the electrochemical cell by electrical means.
Secondary cells can be recharged; that is, they can have their chemical reactions reversed by supplying electrical energy to the cell, restoring their original composition.
Primary electrochemical cells
Primary electochemical cells can produce current immediately on assembly. Disposable cells are intended to be used once and discarded. Disposable primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms.
Common types of disposable cells include zinc-carbon cells and alkaline cells. Generally, these have higher energy densities than rechargeable cells, but disposable cells do not fare well under high-drain applications with loads under 75 ohms (75 Î©).
Secondary electrochemical cells
Secondary electrochemical cells must be charged before use; they are usually assembled with active materials in the discharged state. Rechargeable electrochemical cells or secondary electrochemical cells can be recharged by applying electric current, which reverses the chemical reactions that occur during its use. Devices to supply the appropriate current are called chargers or rechargers.
The oldest form of rechargeable cell is the lead-acid cell. This electrochemical cell is notable in that it contains a liquid in an unsealed container, requiring that the cell be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen gas produced by these cells during overcharging. The lead-acid cell is also very heavy for the amount of electrical energy it can supply. Despite this, its low manufacturing cost and its high surge current levels make its use common where a large capacity (over approximately 10Ah) is required or where the weight and ease of handling are not concerns.
An improved type of liquid electrolyte cell is the sealed valve regulated lead acid (VRLA) cell, popular in the automotive industry as a replacement for the lead-acid wet cell. The VRLA cell uses an immobilized sulphuric acid electrolyte, reducing the chance of leakage and extending shelf life. VRLA cells have the electrolyte immobilized, usually by one of two means:
Gel cells contain a semi-solid electrolyte to prevent spillage.
Absorbed Glass Mat (AGM) cells absorb the electrolyte in a special fibreglass matting
Other portable rechargeable cells are (in order of increasing power density and cost): nickel-cadmium cells (NiCd), nickel metal hydride cells (NiMH), and lithium-ion cells(Li-ion). By far, Li-ion has the highest share of the dry cell rechargeable market. Meanwhile, NiMH has replaced NiCd in most applications due to its higher capacity, but NiCd remains in use in power tools, two-way radios, and medical equipment.
Electrochemical Cells Galvanic and Electrolytic Cells
Oxidation-reduction or redox reactions take place in electrochemical cells. There are two types of electrochemical cells. Spontaneous reactions occur in galvanic (voltaic) cells; nonspontaneous reactions occur in electrolytic cells. Both types of cells contain electrodes where the oxidation and reduction reactions occur. Oxidation occurs at the electrode termed the anode and reduction occurs at the electrode called the cathode.
Human Stem Cell Research and Regenerative Medicine
Regenerative medicine is a multidisciplinary field concerned with the replacement, repair or restoration of injured tissues. This field emerged from the need for reconstruction in children and adults in whom tissue has been damaged by diseases, trauma and congenital anomalies. Stem cell research is a promising field with an alluring potential for therapeutic intervention, and thus begs a critical understanding of the long-term consequences of stem cell replacement. Stem cells have unrestricted potential to divide and this strength is used for the regeneration and repair of cells within the body during tissue damage. Research on stem cells is advancing knowledge about how an organism develops from a single cell and how healthy cells replace damaged cells in adult organisms. This promising area of science is also leading scientists to investigate the possibility of cell-based therapies to treat disease. In our present review we tried to provide the information about stem cells and their significant role in regenerative medicine for treatment of various diseases.
Keywords: Stem cells – Embryonic stem cells – Adult stem cells – Stem cells treatment – Regenerative medicine – stem cell therapy.
INTRODUCTION: Regenerative medicine is an emerging and rapidly evolving field of research and therapeutics to restore, maintain and improve body functions (1). Daar and Greenwood (2) stated that regenerative medicine aims at ‘repair, replacement or regeneration of cells, tissue or organs to restore impaired function’. It aids the body to form new functional tissue to replace lost or defective tissue. Ultimately, this will help to provide therapeutic treatment for conditions where current therapies are inadequate. Cell therapy and tissue engineering are part of the broader field of regenerative medicine, whose aim is the delivery of safe, effective and consistent therapies. The human body has an endogenous system of regeneration and repair through stem cells, where stem cells can be found almost in every type of tissue. This process is highly evolved through evolution, and so it is logical that restoration of function is best accomplished by these cells. Therefore, stem cells hold great promise for the future of translational medicine. Regenerative medicine is also a primer for pediatricians (3-6).
In the early 1900’s European researchers realized that the various type of blood cells – white blood cells, red blood cells and platelets all came from a particular ‘stem cell’. Stem cells were first studied by Becker et al (7), who injected bone marrow cells into irradiated mice and noticed that nodules developed in the spleens of the mice in proportion to the number of bone marrow cells injected. They concluded that each nodule arose from a single marrow cell. Later on, they found by evidence that these cells were capable of infinite self-renewal, a central characteristic of stem cells. Thus, stem cells by definition have two essential properties, i.e. the capacity of self renewal, and the capacity to differentiate into different cell lineages. Under the right conditions, or given the right signals, stem cells can give rise (differentiate) to the many different cell types that make up the organism (Fig-1). Stem cell lineage determination is explained by several ideas, one among is focused on the stem cells microenvironment or ‘niche’. A niche consists of signalling molecules, intercellular communication and the interaction between stem cells and their neighboring extracellular matrix. This three-dimensional microenvironment is thought to influence/control genes and properties that define ‘stemness’ of the stem cells, i.e. self-renewal or development to committed cells. An interesting theory put forward is that stem cells might be terminal differentiation cells with the potential to display diverse cell types, depending on the host niche. Adult stem cells that are implanted into a totally different niche (different germ layer) can potentially differentiate into cell types similar to those found in the new environment. The potential of stem cells and its plasticity are having invaluable properties for regenerative medicine (8). Beneficiaries of regenerative medicine include the increasingly ageing population, people with sports injuries and war casualties. The tremendous technological progress achieved during the last decade in gene transfer methods and imaging techniques, as well as recent increases in our knowledge of cell biology, have opened new horizons in the field of regenerative medicine. Genetically engineered cells are a tool for tissue engineering and regenerative medicine, albeit a tool whose development is fraught with difficulties (9,10). This review summarizes current knowledge of stem cells in regenerative medicine particularly in the treatment of various diseases.
TYPES OF STEM CELLS:
There are two main types of stem cells, embryonic and non-embryonic. Embryonic stem cells (ESCs) are totipotent and, accordingly, they can differentiate into all three embryonic germ layers. On the other hand, non-embryonic stem cells (non-ESCs), also known as adult stem cells, are just multipotent; their potential to differentiate into different cell types seems to be more limited (11). Embryonic stem cells are derived from the inner cell mass of a blastocyst (a very early embryo) and the adult stem cells are derived from mature tissue. A large variety of cell types have been used for regenerative medicine, including adult cells, resident tissue specific stem cells, bone marrow stem cells, embryonic stem cells(12) and the recent breakthrough discovery of induced pluripotent stem cells from mature/adult cells (iPS) (13).
EMBRYONIC STEM CELLS:
Human embryonic stem cells (ES cells) are primitive (undifferentiated) cells that can self-renew or differentiate into all cell types found in adult human body (14,15). The derivation of mouse ES cells was first reported in 1981 (16,17) but it was not until 1998 that the derivations of human ES cell lines were first reported (18). A new era in stem cell biology began in 1998 with the derivation of cells from human blastocysts and fetal tissue with the unique ability of differentiating into cells of all tissues in the body. Embryonic stem cells are derived from embryos at a developmental stage before the time that implantation would normally occur in the uterus. Each of the cells (blastomeres) of these cleavage-stage embryos is undifferentiated. The first differentiation event in humans occurs at approximately five days of development, when an outer layer of cells committed to becoming part of the placenta (trophectoderm) separates from the inner cell mass (ICM). The ICM cells have the potential to generate any cell type of the body, but after implantation, they are quickly depleted as they differentiate to other cell types with more limited developmental potential. The ICM derived cells can continue to proliferate and replicate them indefinitely and still maintain the developmental potential to form any cell type of the body (Fig-1).
Bongso et al. (19) first described isolation and culture of cells of the inner cell mass of human blastocysts, and techniques for deriving and culturing stable hES cell lines were first reported in 1998 (18). The trophectoderm was removed from 5th day blastocysts consisting ICM of 30-34 cells, was placed into tissue culture. The possible sources of stem cells are embryos created via In vitro Fertilization (IVF) (20), embryos or fetuses obtained through elective abortion and embryos created via somatic cell nuclear transfer (SCNT) or cloning. They can be isolated by immunosurgery from the inner cell mass of the embryo during the blastocyst stage, and are usually grown on feeder layers consisting of mouse embryonic fibroblasts or human feeder cells (21). More recent reports have shown that these cells can be grown without the use of a feeder layer (22), and thus avoid the exposure of these human cells to mouse viruses and proteins. These cells have demonstrated longevity in culture by maintaining their undifferentiated state for at least 80 passages when grown using published protocols (3, 23). The source of ESCs opens a Pandora’s box of ethical dilemmas, including the moral status of the embryo, the sanctity of life (24) and the possible use of saviour siblings as a source of ESCs. These add to the long-standing accusation to scientists of tampering with the natural process of life. The ethical debate relates to whether it is right to use human tissue in an abnormal manner. Life-saving situations where the strongest ethical arguments can be made to support the use of cells that are from an embryo that will not become an independent human life. As non-ESCs use becomes more widespread, then acceptance of ESCs treatments may increase. When ethical obstacles are overcome, ESCs might be introduced for treating several conditions, including diabetes (25), spinal cord injuries (26) and liver (27) and heart transplantation (28). Recently Guenou et al (29) demonstrated that human embryonic stem cells (hESCs) can differentiate into mature keratinocytes able to generate a pluristratified epithelium on immunodeficient mice. Jukes et al (30) reviewed on chondrogenic and osteogenic differentiation of mouse and human embryonic stem cells (ESCs) and their potential in cartilage and bone tissue engineering.
Embryonic stem cells have been shown to differentiate into cells from all three embryonic germ layers in vitro (Fig-1). Skin and neurons formed from ectodermal differentiation (31-34), blood, cardiac cells, cartilage, endothelial cells, and muscle formed from mesodermal differentiation (35-37) and pancreatic cells from endodermal differentiation (38). In addition, as further evidence of their pluripotency, embryonic stem cells can form embryoid bodies, the cell aggregations that contain all three embryonic germ layers, while in culture, and can form teratomas in vivo (39,40).
HUMAN EMBRYONIC GERM CELLS:
Embryonic germ cells are derived from primordial germ line cells in early fetal tissue. Unlike embryonic stem cells, animal experiments on embryonic germ cells have been limited. In 1998 the isolation, culture, and partial characterization of germ cells derived from the gonadal ridge of human tissue obtained from abort uses were reported (41). There are fewer data from animal embryonic germ cell experiments than from ES cell experiments, but it is generally assumed that the range of potential fates will be relatively limited compared to ES cells, because the embryonic germ cells are much further along in development (5-9 weeks). Fetal tissue may provide committed progenitors, but the feasibility of large scale sourcing and manufacturing of products utilizing such cells is questionable. Furthermore, the behavior of these cells in vivo is not well understood; significant research will be required to avoid unwanted outcomes, including ectopic tissue formation i.e., additional, unwanted tissue, tumor induction, or other abnormal development (31).